libelf: Make all callers call elf_check_broken

This arranges that if the new pointer reference error checking
tripped, we actually get a message about it.  In this patch these
messages do not change the actual return values from the various
functions: so pointer reference errors do not prevent loading.  This
is for fear that some existing kernels might cause the code to make
these wild references, which would then break, which is not a good
thing in a security patch.

In xen/arch/x86/domain_build.c we have to introduce an "out" label and
change all of the "return rc" beyond the relevant point into "goto
out".

This is part of the fix to a security issue, XSA-55.

Differences in 4.1 backport:
 * No xen/arch/arm.
 * There was less error handling in xen/arch/x86/domain_build.c
   so less need to change it.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>

3 files changed, 27 insertions, 3 deletions

diff --git a/tools/libxc/xc_dom_elfloader.c b/tools/libxc/xc_dom_elfloader.c
index 945df7a2aa..e733afd16e 100644
--- a/tools/libxc/xc_dom_elfloader.c
+++ b/tools/libxc/xc_dom_elfloader.c
@@ -276,6 +276,13 @@ static int xc_dom_load_elf_symtab(struct xc_dom_image *dom,
             elf_store_field(elf, shdr, e32.sh_name, 0);
     }
 
+    if ( elf_check_broken(&syms) )
+        DOMPRINTF("%s: symbols ELF broken: %s", __FUNCTION__,
+                  elf_check_broken(&syms));
+    if ( elf_check_broken(elf) )
+        DOMPRINTF("%s: ELF broken: %s", __FUNCTION__,
+                  elf_check_broken(elf));
+
     if ( tables == 0 )
     {
         DOMPRINTF("%s: no symbol table present", __FUNCTION__);
@@ -312,13 +319,16 @@ static int xc_dom_parse_elf_kernel(struct xc_dom_image *dom)
     {
         xc_dom_panic(dom->xch, XC_INVALID_KERNEL, "%s: ELF image"
                      " has no shstrtab", __FUNCTION__);
-        return -EINVAL;
+        rc = -EINVAL;
+        goto out;
     }
 
     /* parse binary and get xen meta info */
     elf_parse_binary(elf);
     if ( (rc = elf_xen_parse(elf, &dom->parms)) != 0 )
-        return rc;
+    {
+        goto out;
+    }
 
     /* find kernel segment */
     dom->kernel_seg.vstart = dom->parms.virt_kstart;
@@ -331,7 +341,13 @@ static int xc_dom_parse_elf_kernel(struct xc_dom_image *dom)
     DOMPRINTF("%s: %s: 0x%" PRIx64 " -> 0x%" PRIx64 "",
               __FUNCTION__, dom->guest_type,
               dom->kernel_seg.vstart, dom->kernel_seg.vend);
-    return 0;
+    rc = 0;
+out:
+    if ( elf_check_broken(elf) )
+        DOMPRINTF("%s: ELF broken: %s", __FUNCTION__,
+                  elf_check_broken(elf));
+
+    return rc;
 }
 
 static int xc_dom_load_elf_kernel(struct xc_dom_image *dom)
diff --git a/tools/libxc/xc_hvm_build.c b/tools/libxc/xc_hvm_build.c
index e3efca98e8..6c4e41e5ea 100644
--- a/tools/libxc/xc_hvm_build.c
+++ b/tools/libxc/xc_hvm_build.c
@@ -393,11 +393,16 @@ static int setup_guest(xc_interface *xch,
         munmap(page0, PAGE_SIZE);
     }
 
+    if ( elf_check_broken(&elf) )
+        ERROR("HVM ELF broken: %s", elf_check_broken(&elf));
+
     free(page_array);
     return 0;
 
  error_out:
     free(page_array);
+    if ( elf_check_broken(&elf) )
+        ERROR("HVM ELF broken, failing: %s", elf_check_broken(&elf));
     return -1;
 }
 
diff --git a/tools/xcutils/readnotes.c b/tools/xcutils/readnotes.c
index d462f80245..b940a36722 100644
--- a/tools/xcutils/readnotes.c
+++ b/tools/xcutils/readnotes.c
@@ -224,6 +224,9 @@ int main(int argc, char **argv)
 		printf("__xen_guest: %s\n",
                        elf_strfmt(&elf, elf_section_start(&elf, shdr)));
 
+	if (elf_check_broken(&elf))
+		printf("warning: broken ELF: %s\n", elf_check_broken(&elf));
+
 	return 0;
 }